1. Field of the Invention
Embodiments of the present invention generally relate to techniques for inspecting objects and, more particularly, to an automated optical inspection technique and system utilizing multiple objectives per camera.
2. Description of the Related Art
Automated optical inspection (AOI) systems are used to inspect a wide variety of objects, such as semiconductor wafers and printed circuit boards (PCBs), for defects. Such systems typically utilize one or more image acquisition or “microscope” modules to capture images that cover the entire surface area of the object that is to be inspected. These images are then fed to some type of computer system for processing using various types of algorithms design to identify defects in the object.
Typically, each image scanning module includes an illumination source to illuminate the portion of the article under inspection, some type of front lens assembly (referred to as an objective) that guides light back to an image capture device, such as a charge-coupled device (CCD) camera. One common type of AOI system utilizes a scanning approach, whereby the inspected article is divided into (e.g., horizontal) strips and a single image scanning module is moved back and forth, collecting images for successive strips on each pass. This approach may work well in some applications, for example, where the total surface area being inspected is relatively small and the total processing throughput requirements are relatively low. However, for applications requiring higher throughput, such as web inspection, which may have relatively large surface areas to inspect (e.g., with horizontal dimensions of several feet), a scanning approach may take too much time.
These higher throughput applications typically require multiple image scanning modules, with a camera and objective per module, allowing multiple images to be processed in parallel leading to increased throughput. In general, the number of image scanning modules utilized depends on various system requirements, as well as the individual image scanning module specifications. For example, the number of image scanning modules required is generally proportional to the total surface area to inspect and the required throughput of the system, and inversely proportional to the field of view (FOV) of each objective and pixel rate of the camera. Unfortunately, high resolution cameras with high pixel rates tend to be expensive, often costing many times more than the objectives. As a result, while systems utilizing multiple image scanning modules may achieve higher throughput than systems that scan with a single module, the cost of multiple high resolutions cameras may render such systems cost prohibitive.
Accordingly, a need exists for a system and technique for automated optical inspection resulting in high throughput relative to conventional scanning systems and low cost relative to systems that utilize a single objective per camera.